Hydraulic power system control



Jan 7, J. L GRATZMULLER HYDRAULIC: POWER SYSTEM coN'rRL l' Filed Nov. zo, 1961 I 2 sheets-sheet 1y FIG. 2

FIGAh EXHAUST LINE 4 Jan. 7,` 1964 J. GRATZMULLER HYDRAULIC POWER SYSTEM CONTROL Filed Nov. 20, 1961 FIG-4 2 Sheets-Sheet 2 Line Acc umulator United States Patent O 3,116,607 HYDRAULIC POWER SYSTEM CONTROL .lean Louis Gratzmuller, 66 Boulevard Maurice Barrs, Neuilly sur Seine, France Filed Nov. 20, 1961, Ser. No. 153,447 Claims priority, application France Nov. 28, 1960 11 Claims. (Cl. 60-51) This invention relates to the control of a hydraulic power system comprising la service line, an exhaust port or line and a hydraulic accumulator, preferably but not necessarily hydro-pneumatic, having a free piston, the control system or device enabling the service line to be connected selectively to the exhaust line or to the accumulator.

The invention is more especially (but not exclusively) adapted to systems in which the service line is connected to one or more single-acting hydraulic actuators with resilient, e.g., spring, return means and has been more especially devised to meet the requirements of an actuator, whose power stroke (under hydraulic pressure) must be completed without fail when once initiated and whose return stroke (under the effort of the resilient means) must not only be completed without fail but must be performed very rapidly, the exhausting of liquid from the actuator lbeing in no way impeded. Such requirements are essential if the actuator is coupled to an electrical circuit-breaker or like device, whose contact-making stroke is effected by the power stroke of the actuator and whose contact-breaking stroke is effected by the return stroke of the actuator.

A further requirement, particularly for circuit-breaker operations, is that, once the actuator has completed its stroke in either direction, it must remain in its iinal position until the reverse action is initiated by human or other external agency (e.g. an automatic device responsive to overload or short-circuit in the case of a circuit-breaker).

Retention of the actuator in its end of return stroke position may be ensured by providing that the venting of the service line to exhaust can only be terminated by positive, external control action; and retention of the actuator in its end of power stroke position by providing that the service line remains in communication with the accumulator :and therefore under pressure, until the service line is vented to exhaust by positive, external control action.

However, even if these conditions are met, malfunctioning of the system can occur, with possibly disastrous consequences in the case of circuit-breaker operation, as a result of loss of liquid from the hydraulic system leading to an inadequate reserve of liquid therein and an object of the invention herein is -to eliminate such malfunctioning.

A further object of the invention is the provision, in combination with a hydraulic power system as rst herein dened, of a control device comprising a valve assembly having two settings, selectable by external agency, in one of which the service line is connected to exhaust, the accumulator being isolated, and in the other with the accumulator, the exhaust port or line being isolated, and a third or intermediate setting, in which the service line, and the accumulator are both connected to exhaust, and means actuated by the free piston of the accumulator on reaching Ia predetermined position in its liquid-expelling stroke for automatically bringing the valve assembly momentarily into said third setting and thereafter causing it, either by direct action or indirectly, to assume its first-mentioned setting, enabling the accumulator to be 1re-charged; the said position of the free piston being determined by the consideration of preventing the reserve of liquid in the accumulator from vanishing altogether or becoming too low for safety.

How these objects, and such others as may hereinafter appear, may be achieved, and the nature of the invention generally will be more easily understood from the following description referring to the accompanying drawings illustrating embodiments of the invention, by way of example only :and without implied limitation of the scope of the invention, which is dened in the hereto appended claims.

In the drawings,

FIGURES 1 to 3 illustrate schematically, partially in section and highly simplified, an embodiment of the invention in three possible settings of the valve assembly; and

FIGURE 4 illustrates in greater detail one practical embodiment of lthe invention in axial section.

It is to be understood that FIGURES l to 3 are intended to demonstrate the main features of the invention and its operation, all subordinate detail being omitted and to the form of the elements that are illustrated having been selected with a view to simplicity Iand clarity of the drawings rather than ultimate practical suitability.

FIGURE 4, on the other hand, is fully detailed. Reference characters common to FIGURE 4 and the other three figures identify the same or fully equivalent elements.

Referring to FIGURES l to 3, 1 is the accumulator and 2 is its free piston, `above which is the liquid chamber, 3 is the service line and 4 an exhaust line. 5 is the body of the valve assembly, which is of the sliding piston type comprising two slidable closure members or lands 6 and 7 rigidly interconnected by an axial rod. Land 6 obstructs or exposes a port leading (via a duct 11) to the liquid chamber of the accumulator and land 7 obstructs or exposes an exhaust port leading to the exhaust line 4. The service line 3 terminates in the valve body at a port, which is never obstructed, communicating with the space between lands 6 and 7.

Rigidly connected to the rod uniting lands 6 and 7 is the piston 8 of a single-acting hydraulic jack 9, whose working chamber is above the piston, which is loaded from below by a return spring 10. The working chamber of jack 9 is connected to duct 11 (and thereby to the accumulator) by a duct 12, in which is a manually operable cock 15, and to the exhaust line 4 by a duct 14 (branched from duct 12) in which is a manually operable cock 16, and to the service line 3` by a duct 13, in which is a calibrated restrictor 17.

The rod interconnecting the lands 6 :and 7 is extended downwardly by a push rod 18, which extends, through a clearance opening in 4the head of the accumulator 1, into the liquid chamber of the accumulator to be met by the free piston 2 when it rises above a predetermined level (see FIGURE 3).

The control device illustrated in FIGURES l to 3 operates as follows:

The service line 3 is connected to the working chamber of a single-acting cylindrical hydraulic actuator 40 having a piston 41 therein normally biased toward the cylinder inlet opening by spring 42. FIGURE 2 shows the situation when the last-mentioned actuator is not energised, the service line being connected to the exhaust line, and therefore not pressurized, and the accumulator being isolated. The jack 9 is also de-energised, since its working chamber communicates via duct 13 and the service line El` with the exhaust line 4 and is therefore at exhaust pressure, the cock 15 being closed. In this situation cock 16 will be kept closed.

To pressurize the service line 3, the cock 15 is temporarily opened to admit liquid under the accumulator pressure to the jack 9 via duct 12. The restrictor 17, by preventing rapid escape of liquid from the jack 9 via duct 13 into the as yet unpressurized service line, maintains the pressure in jack 9 causing the piston 8 to descend against the effort of spring 10 and bring the valve lands 6 and 7 into the setting of FIGURE 1, isolating the exhaust line 4 and connecting the service line 3 to the accumulator via duct 11. The service line will now be under accumulator pressure and this pressure will be transmitted, via duct 13, to the working chamber of jack 9 maintaining the piston S and valve lands 6, 7 in the position of FIGURE. 1, whereupon the cock 1S is closed.

The service line =3 will therefore continue to be pressurized and the actuator 4t) connected thereto energised until, in normal circumstances, cock 16 is opened to connect the Working chamber of jack 9 to the exhaust line via duct 14, thus relieving the pressure in jack 9 and allowing spring 1@ to return the piston 8 and lands 6, 7 to the position of FIGURE 2. Until this position is reached the service line 3 will remain under pressure and liquid will flow from it into the working chamber of jack 9 via duct 13; but since this flow takes place through the restrictor 17, the latter imposes a large pressure drop between the service line 3 and the working chamber of jack 9, so that the relief of pressure in the latter is substantially unaffected and the rapid return of piston 8 by spring 10 to the position of FIGURE 2 is unimpeded. During this return movement, before land 6 isolates the accumulator, loss of liquid from the latter via duct 14 and cock 16 is prevented because cock 15 has already been closed.

Now, in the performance of the complete cycle of operations as above described a quantity of liquid corresponding to the capacity of the actuator 40 connected to the service line is expelled from the system via the exhaust line, so that the accumultaor piston 2 rises to a corresponding extent.

Consequently, after a certain number of complete cycles have been performed the piston 2 will reach a position in which (unless the accumulator is re-charged in the meanwhile) the reserve of liquid in it is insuicient for the completion of the first phase of another cycle in which the actuator 43 connected to the service line is being charged, so that this actuator will be unable to complete its stroke.

Moreover, if there is slow leak in the hydraulic circuit, the piston Z will rise slowly all the time and not only during the charging of the last-mentioned actuator with consequent diminution of the reserve of liquid in the system; and if while the actuator is still charged, the device being in the condition shown in FIGURE 1, the piston 2 actually reaches the end of its stroke so that the accumulator can no longer make good the leakage loss, the actuator will discharge slowly through the leak and the apparatus it operates will return slowly to its end of cycle position. If the operated apparatus is a circuit-breaker and discharge of the actuator operating it effects or allows the circuit-breaking movement, such slow discharge of the actuator would have disastrous consequences.

The device as illustrated forestalls the possibility of unforeseen reduction of the reserve of liquid in the system to a dangerously low level by ensuring that, when the level of liquid reserve approaches or reaches such a low value, the service line is automatically vented to exhaust, thus rapidly discharging the actuator connected to the service line.

When the piston 2 has risen to a predetermined level at which the liquid reserve is too low, it encounters the tip of the push rod 18. Further rise of the piston 2 pushes back the push rod and with it the lands 6, 7 until the position of FIGURE 3 is reached, in which land 7 has exposed the port leading to the exhaust line 4 but land 6 has not yet covered the port leading to duct 11, thereby venting both the service line 3 and the accumulator 9 to exhaust. The pressure in the service line is thus promptly relieved enabling the actuator connected thereto to discharge rapidly. At the same time the pressure on piston 8 transmitted from the service line Va duct 13 is also relieved enabling the spring 10,

assisted by continued upward movement of piston 2, no longer resisted by hydraulic pressure, since the liquid chamber of the accumulator is also vented to exhaust, to complete the upward movement of piston 8 and lands 6, 7 into the position of FIGURE 2, in which the service line is still vented to exhaust, but the accumulator is sealed by land 6. However, by the time this position is reached the piston 2 will have completed its upward stroke and expelled the remaining reserve of liquid, so that inadvertent opening of cock 15 cannot start a new cycle of operation. The whole system therefore remains incapable of further operation until the accumulator has been re-charged with liquid, which can be effected without diiculty and without inadvertently re-starting the operative cycle, provided cock 15 is kept closed. Once the accumulator is re-charged, the operative cycle can be re-started without any additional re-cocking action.

In the embodiment illustrated in FIGURE 4, the sliding valve closure members of FIGURES 1 to 3 are replaced by seatable valve members, such as cones or balls, having a very small travel between their closed and fully open positions, thus promoting promptness of response of the device to control signals.

This embodiment comprises, as does that of FIGURES 1 to 3, a hydro-pneumatic accumulator having a cylinder 1 and free piston 2 with the liquid space above it, a connection for a service line 3, a valve body 5, valve closure members 6 and 7, a valve operating single-acting jack having a cylinder 9, a piston 8 and a return spring 10, a duct 12 connecting the jack cylinder 9 to the accumulator and containing a cock 15 and a duct 13 connecting the jack cylinder to the service line and containing a calibrated restrictor 17.

In this embodiment the exhaust line or channel 4 of FIGURES 1 to 3 and the duct 11 of FIGURES 1 to 3 connecting the accumulator with the valve port controlled by member 6 are respectively replaced by annular clearances around the valve body as hereinafter explained. Moreover, the valve members 6 and 7 are not rigidly interconnected.

11n the construction of FIGURE 4, the head of the accumulator is formed by a block 19 into which the cylinder 1 is screwed; and on top of block 19 is mounted casing 24 constituting a reservoir of liquid at low pressure, being vented at the top to atmosphere or a lowpressure region by means not illustrated. The valve body 5 is mounted in a Huid-tight manner in a central bore of the block 19, being secured therein by a ring nut 37, and extends downwardly into the accumulator cylinder 1 and upwardly into the reservoir 24.

The valve body has a straight-through axial bore 20 of varying diameter, the portions of enlarged diameter serving as guides for the cone valve members 6 and 7 and the shoulderings of this bore forming seatings for these valve members. In the valve body are transverse passages 21, 22, 23, respectively providing communication between the bore 2() and the service line connection 3, an annular clearance round the valve body communicating with the liquid chamber of the accumulator, and an annular clearance (through nut 37) communicating with the reservoir 24.

Communication between passage 21 and passage 23 is controlled by valve member 7, which is integral with the piston 8; and communication between passage 21 and passage 22 is controlled by valve member 6, which is not integral with valve member 7 but is seatable by a spring 26 and unseatable by a push rod 25 forming an extension of valve member 7. In the position shown in FIGURE 4 (corresponding to the valve-setting of FIG- URE 2) member 7 is fully unseated and member 6 is seated, thus putting the service line connection 3 into communication with the exhaust reservoir 24 and isolating the accumulator 1. In this position there is a very small clearance between the push rod 25 and member 6, suiiicient to allow the spring 26 to seat the member 6 truly.

The push member 18, which functions in the same way as the rod 18 of FIGURES 1 to 3, is not attached to the valve member 6, but is in the form of a push button slidable in the open, lower end of bore 20, in which it is trapped by a circlip 34. Push member 18 also has an upward extension forming the abutment of spring 26. In the position shown in FIGURE 4, spring 26 has seated valve member 6 and presses the push member 18 against its retaining circlip 34.

To complete the description of the structure of FIG- URE 4, the valve body is prolonged upwardly by a screwed-on extension having an axial bore 27 communicating with the working chamber of the jack cylinder portion 9 and housing the spring 10. Duct 12 communicates with the bore 27 through a non-return, ball valve 28; and bore 27 communicates with a conventional safety valve 31 loaded by a calibrated spring 32. Valve 31 vents the bore 27 to exhaust when the hydraulic pressure on the jack piston 8 exceeds a predetermined maximum value. In a modified form of construction valve 31 is omitted for reasons hereinafter explained.

The upper end of bore 27 is closed by a plug 29 having an axial bore communicating, via a side passage 35 with the exhaust reservoir 24, and terminating at its lower end in an opening communicating with bore Z7. When pressure in bore 27 exceeds the exhaust pressure, this opening is sealed by a ball 38, which is unseatable by a push rod 30 or by gravity in the absence of pressure in bore 27. Rod 30 is normally retracted by a spring 39, allowing ball 38 to be seated, but can be depressed, against the efort of spring 39, to unseat ball 38, by an electromagnet diagrammatically represented by a coil 36. The passage 35 is the equivalent of duct 14 of FIGURES 1 to 3; and the ball valve assembly 38, 3i), 39, 36 is the equivalent of cock 16 of FIGURES 1 to 3.

The upward stroke of the accumulator piston 2 is limited by abutment against the lower end 33 of the valve body 5. When the piston 2 is just touching the push member 18 without raising it from contact with its retaining circlip 34, as shown in FIGURE 4, the clearance between the piston and its tinal abutment 33 is about half the total travel, between their fully unseated and seated positions, of the valve members 6 and 7 for reasons hereinafter explained.

The operation of the embodiment of FIGURE 4 is as follows:

In the position shown in the ligure (corresponding to that of FIGURE 2) ball 38 is unseated and cock 15 is assumed closed, so that the bore 27 and the working chamber of jack 9, 8 are vented to exhaust and isolated from the accumulator (which is assumed to be charged). Piston 8 is therefore not under hydraulic pressure and spring 1t) has therefore retracted it and valve member 7 to unseat the latter and establish communication between the service line connection 3 and the exhaust reservoir. At the same time the push rod 25 is retracted, allowing spring 26 to seat the valve member 6, thereby isolating the service line connection 3 from the accumulator. Moreover, the working chamber of jack 9, 8 is then also vented to exhaust via duct 13 and passages 21, 2t) and 23. Since the service line is vented to exhaust, a single-acting hydraulic actuator connected thereto will therefore be discharged.

On momentarily opening the cock duct 12 is pressurized from the accumulator, the non-return ball valve 28 is unseated and the contents of bore 27 and the working chamber of jack 9, 8 pressurized, so that piston 8 is driven down (against the effort of spring 10) to seat valve member 7 and, through the action of push rod 25, to unseat valve member 6 against the effort of spring 26, thus putting the service line into communication with the accumulator and isolating it from exhaust. The service line is therefore pressurized and the pressuretherein is transmitted via duct 13 to the jack 9, 8 to retain the valve members 6 and 7 in their new positions (corresponding to that of FIGURE 1) after closure of cock 15. Maintenance of pressure in the service line therefore ensures completion of the power stroke of the actuator connected to the service line and maintenance thereof in its end of power stroke state until the control device of FIGURE 4 receives a further control signal.

Normally such further signal will be given by momentary excitation of electromagnet 36 to unseat ball 38 and vent the jack 9, S to exhaust via passage 35, whereupon the system reverts to the condition shown in the figure and ball 3S will remain unseated, since there is no longer any pressure in bore 27 to hold it on its seat, notwithstanding cessation of excitation of the electromagnet 36. This completes the normal cycle of the device.

If, however, before the ball 38 is unseated by excitation of the electromagnet 36, the accumulator piston 2 reaches the position in its upward stroke at which it encounters the push member 18 as shown in the gure, any further upward movement of the piston 2 will push the member 1S upwards and this upward movement will be transmitted through spring 26 to the valve member 6 and therefrom through push rod 25 to valve member 7 unseating the latter and at the same time slightly raising the piston Ei, causing a temporary increase of pressure in chamber 27. In practice, the decrease of volume of chamber 27 is very slight, due to the short travel of piston 8; and consequently the temporary increase of pressure in chamber 27 is also slight, with the result that, usually, valve member 7 becomes unseated before the safety valve 31 can act. For this reason it is in many cases permissible to omit the safety valve altogether without detriment.

The movement will continue until the piston 2 meets its abutment 33, but since the clearance between the piston 2, when it rst meets the member 18, and its abutment 33 is less than the total travel of the valve members, the Valve member 6 will not be seated when the piston 2 meets its abutment, so that both valve members 6 and 7 will be unseated and both the service line and the accumulator will be vented via bore 2t? and passage 23 to exhaust, the former via passage 21, and the latter via passage 22. In this way the pressure in the whole system is totally relieved, enabling the actuator connected to the service line to be rapidly discharged by its spring return means, and enabling the spring 1t) to complete the upward travel of the piston 8, valve member 7 and push rod 25, thus allowing the valve member 6 to be fully seated by spring 26 and the ball 38 to drop olf its seating. The system is thus restored to the condition shown in FIGURE 4 and once the accumulator has been recharged with liquid, which operation can be effected without difculty, the accumulator being now isolated, a new cycle of operation can be initiated by opening cock 15, no additional re-cocking action being needed. For simplicity, the necessary connection for re-charging the accumulator from a pump or the like, which may be of conventional type, has been omitted from the figure.

It will be noted that the valve body 5 extends far enough into the accumulator cylinder 1 to ensure that the piston 2 meets the push member 18 and thereafter its abutment 33 before the reserve of liquid in the accumulator is entirely exhausted; also that the construction adopted enables the accumulator cylinder 1 (with its piston 2) to be unscrewed and removed for replacement, if desired by a cylinder of different capacity, without disturbing the rest of the system.

The details of the construction illustrated in FIGURE 4 have been devised to simplify machining operations and to facilitate assembly and dismantling, as will be evident from inspection of the ligure.

It is to be understood that the invention is not limited to the embodiments described with reference to and illustrated in the accompanying drawings, but includes all such modifications and variants as may be within the compctance of those skilled in the art and lie within the spirit and scope of the invention as deiined in the hereto appended claims.

Having thus described the invention, what is claimed as new and desired to be secured by Letters Patent is:

l. In combination with a hydraulic power system including a service line, an exhaust line and a hydropneumatic accumulator having a free piston, a control device comprising valve means movable between an exhaust position, in which said valve means establishes communication between said service line and said exhaust line; a supply position, in which said valve means establishes a communication between said service line and said accumulator, and an intermediate position, in which said valve means establishes a communication between said exhaust line and both said service lines and accumulator; controllable means for selectively moving said valve means between said supply position and said exhaust position, said device further comprising a movable mechanical element operative on the valve means and extending into the accumulator and so actuabie by the said free piston when the latter reaches a predetermined position near the end of its liquid expelling stroke to move the valve means from said supply position to said intermediate position.

2. In combination with a hydraulic power system including a service line, an exhaust line, and a hydraulic accumulator having a free piston and a fiuid outlet line, a control device including a chamber into which one end of each of said lines open, a Valve assembly within said chamber for controlling the openings into said lines, means operative on the valve assembly and controllable by an external agency for selecting valve assembly settings, in the first of which the service line communicates with the exhaust line, the accumulator line being closed, and in the second of which the service line communicates with the accumulator line, the exhaust line being closed, and means actuable by the free piston on its reaching a predetermined position in its liquid expelling stroke for temporarily bringing about a third, intermediate valve assembly setting in which the service line, the exhaust line and the accumulator line are all open and in mutual communication, without impeding further movement of the valve assembly from said third to said second setting.

3. In combination with a hydraulic power system including a service line, an exhaust line and a hydraulic accumulator having a free piston and a iiuid outlet line, a control device including a valve assembly into which one end of each of said lines open, for controlling the opening into said lines, means operative on the valve assembly and controllable by an external agency for selecting valve assembly settings, in the first of which the service line communicates with the exhaust line, the accumulator line being closed, and in the second of which the service line communicates with the accumulator line, the exhaust line being closed, and means actuable by the free piston on its reaching a predetermined position in its liquid expelling stroke for temporarily bringing about a third intermediate Valve assembly setting in which the service line, the exhaust line and the accumulator line are all open and in mutual communication, without impeding further movement of the valve assembly from said third to said second setting; said valve assembly comprising two closure members respectively adapted to close the accumulator and the exhaust lines, and the lastmentioned means including a push-member extending into the accumulator for actuation by the free piston thereof.

4. In combination with a hydraulic power system including a service line, an exhaust line and a hydraulic :accumulator having a free piston, a control device including valve means movable, between an exhaust position, in which said valve means establish a communication between said service line and said exhaust line, and a supply position, in which said valve means establish a communication between said service line and said accumulator, through an intermediate position, in which said valve means establish a communication between said exhaust line and both said service line and said accumulator, means actuable by the free piston on its reaching a predetermined position on its liquid discharging stroke moving said valve means from said supply position to said intermediate position, a cylinder, a valve actuating member slidable in said cylinder, resilient means to urge said valve actuating member in a direction in which the latter moves said valve means to said exhaust position, said valve actuating member having a liquid pressure responsive working face to move said valve means under the effect of said liquid pressure against the action of said resilient means to said supply position.

5. The combination defined in claim 4 including further a calibrated restrictor in said third duct means.

6. Hydraulic power apparatus comprising a service line, eg. for connection to a single-acting hydraulic actuator with resilient return means, a hydraulic accumulator having a free piston, a low-pressure liquid reservoir, a valve body extending into the liquid chamber of the accumulator having an axial bore and passages extending from said bore and communicating respectively with said liquid chamber, the reservoir and the service line, said bore having therein two oppositely facing valve seatings, a first valve closure member slidable in said bore and seatable on one of said seatings to isolate the service line from the accumulator, a second valve closure member slidable in said bore and seatabie on the other of said seatings to isolate the service line from the reservoir, a push rod extending from the second said closure member through said bore to make contact with and unseat the first said closure member when the second said closure member is displaced towards its seating and before it reaches such seating, said bore extending beyond the second said valve member to constitute the working chamber of a single-acting hydraulic jack, the piston of which is an integral part of the second said closure member, spring means operative on the second said closure member to unseat it, duct means connecting the accumulator to said working chamber, controllable valve means for selectively opening and closing said duct means, which, when open, admits liquid under pressure to said working chamber to seat the second said closure member against the effort of said spring means, other duct means connecting said working chamber to the service line, a restrictor in said other duct means, control means responsive to an external signal for venting said working chamber to the reservoir, a push member slidably mounted in the axial bore of the valve body and so trapped as normally to extend into the liquid chamber of the accumulator by a distance beyond the end of the valve body less than the total travel of said first and second closure members between their seated and fully unseated positions, and spring means trapped between said push-member and the first said closure member and operative to seat the latter.

7. Hydraulic power apparatus comprising a service line, e.g. for connection to a single-acting hydraulic actuator with resilient return means, a hydraulic accumulator having a free piston, a low-pressure liquid reservoir, a valve body extending into the liquid chamber of the accumulator having an axial bore and passages extending from said bore and communicating respectively with said liquid chamber, the reservoir and the service line, said bore having therein two oppositely facing valve seatings, a first valve closure member slidable in said bore and seatable on one of said seatings to isolate the service line from the accumulator, a second valve closure member slidable in said bore and seatable on the other of said seatings to isolate the service line from the reservoir, a push rod extending from the second said closure member through said bore to make contact with and unseat the first said closure member when the second said closure member is displaced towards its seating and before it reaches such seating, said bore extending beyond the second said valve member to constitute the working chamber of a single-acting hydraulic jack, the piston of which is an integral part of the second said closure member, spring means operative on the second said closure member to unseat it, duct means connecting the accumulator to said working chamber, controllable valve means for selectively opening and closing said duct means, which, when open, admits liquid under pressure to said working chamber to seat the second said closure member against the effort of said spring means, other duct means connecting said working chamber to the service line, a restrictor in said other duct means, said valve body further having a channel connecting said working chamber with the reservoir, a third closure member, e.g. a ball, seatable to obstruct said channel by hydraulic pressure in said working chamber, controllable means for unseating said third closure member, a push member slidably mounted in the axial bore of the valve body and so trapped as normally to extend into the liquid chamber of the accumulator by a distance beyond the end of the valve body less than the total travel of said irst and second closure members between their seated and fully unseated positions and spring means trapped between said push-member and the iirst said closure member and operative to seat the latter.

8. Hydraulic power apparatus comprising a service line, eg. for connection to a single-acting hydraulic actuator with resilient return means, a hydraulic accumulator including a cylinder and a free piston, a block constituting the head of the accumulator into the lower part of which the cylinder is detachably secured, a casing mounted on said block and constituting, with the upper face of said block, a low-pressure liquid reservoir, a valve body secured in said block and extending downwardly into the liquid chamber of the accumulator above the free piston and upwardly into the reservoir, said valve body having an axial bore and passages extending from said bore and communicating respectively with said liquid chamber, the reservoir and the service line, said bore having therein two oppositely facing valve seatings, a first valve closure member slidable in said bore and seatable on one of said seatings to isolate the service line from the accumulator, a second valve closure member slidable in said bore and seatable on the other of said seatings to isolate the service line from the reservoir, a push rod extending from the second said closure member through said bore to make 4 contact with and unseat the rst said closure member when the second said closure member is displaced towards its seating and before it reaches such seating, said bore extending beyond the second said valve member to constitute the working chamber of a single-acting hydraulic jack, the piston of which is an integral part of the second said closure member, spring means operative on the second said closure member to unseat it, duct means connecting the accumulator to said working chamber, controllable valve means for selectively opening and closing said duct means, which, when open, admits liquid under pressure to said working chamber to seat the second said closure member against the eiort of said spring means, other duct means connecting said working chamber to the service line, a restrictor in said other duct means, control means responsive to an external signal for venting said working chamber to the reservoir, a push member slidably mounted in the axial bore of the valve body and so trapped as normally to extend into the liquid chamber of the accumulator by a distance beyond the end of the valve body less than the total travel of said iirst and second closure members between their seated and fully unseated positions and spring means trapped between said push-member and the iirst said closure member and operative to seat the latter.

9. Apparatus as defined in claim 6, in which the duct means connecting the accumulator to said working chamber includes a non-return valve operative to prevent liquid flow from said working chamber towards the accumulator.

10. Apparatus as dened in claim 6, including a springloaded safety valve for venting said working chamber to the reservoir, if and when the hydraulic pressure in said working chamber exceeds a predetermined value.

11. Apparatus as defined in claim 7, in which the controllable means for unseating the third closure member include a second push member, electromagnetic means operative when excited to move said second push member to unseat the third closure member, and spring means operative to retract said second push member when said electromagnetic means is not excited.

Referenees (Cited in the tile of this patent UNITED STATES PATENTS 920,825 Clark May 4, 1909 2,283,516 Tyler a May 19, 1942 2,900,960 Gratzrnuller Aug. 25, 1959 

1. IN COMBINATION WITH A HYDRAULIC POWER SYSTEM INCLUDING A SERVICE LINE, AN EXHAUST LINE AND A HYDROPNEUMATIC ACCUMULATOR HAVING A FREE PISTON, A CONTROL DEVICE COMPRISING VALVE MEANS MOVABLE BETWEEN AN EXHAUST POSITION, IN WHICH SAID VALVE MEANS ESTABLISHED COMMUNICTION BETWEEN SAID SERVICE LINE AND SAID EXHAUST LINE; A SUPPLY POSITION, IN WHICH SAID VALVE MEANS ESTABLISHES A COMMUNICATION BETWEEN SAID SERVICE LINE AND SAID ACVALVE MEANS ESTABLISHES A COMMUNICATION BETWEEN SAID EXHAUST LINE AND BOTH SAID SERVICE LINES AND ACCUMULATOR; CONTROLLABLE MEANS FOR SELECTIVELY MOVING SAID VALVE MEANS BETWEEN SAID SUPPLY POSITION AND SAID EXHAUST POSITION, SAID DEVICE FURTHER COMPRISING A MOVABLE MECHANICAL ELEMENT OPERATIVE ON THE VALVE MEANS AND EXTENDING 